Production of transparent elements such as clearcoats, films, vessels, packaging materials, encapsulation materials, optical fibres, light diffusers or lenses requires materials that are transparent, i.e. have maximum transparency to electromagnetic waves, especially within the spectral range visible to humans from 400 to 800 nm. In a number of applications in the chemical, medical and food and drink sector, transparent elements having high heat distortion and colour stabilities are particularly advantageous. At high typical use temperatures as occur, for example, in the case of use in the presence of chemical processes or in the sterilization of food and drink products and medical products, high-stability transparent materials having good heat distortion and colour stabilities even into the UV-A range are desirable, since thermal yellowing is always the first sign of an unwanted ageing process and degradation process. A further example of the need for transparent and heat- and colour-stable elements is encountered in the field of lighting and solar energy generation, for example in the coating and glazing of photovoltaic solar modules, and in reflective mirror films and Fresnel lenses in concentrator modules which can be exposed to high temperatures frequently and over long periods. Even in the case of many light sources such as incandescent lamps, but also in the case of LEDs, where considerable amounts of heat arise during the generation of light, it is of considerable importance that the transparent elements and the materials used for production thereof have high thermal durability and mechanical stability. Specifically, this means that the materials must not deform or must not become discoloured even in the event of significant heating, for example, since the function of a lens, for instance, can otherwise be lost. At the same time, they also have to have sufficient hardness at normal temperatures, in order that they withstand mechanical stresses.
The increasing use of LEDs as light sources has additionally generated a considerable demand for novel materials which firstly meet the above demands and secondly are suitable for encapsulation or potting of LEDs.
The thesis by Theo Flipsen: “Design, synthesis and properties of new materials based on densely crosslinked polymers for polymer optical fiber and amplifier applications”, Rijksuniversiteit Groningen, 2000 describes the polytrimerization of monomeric HDI with a neodymium/crown ether complex as catalyst. The polyisocyanurate obtained, which is said to have good optical, thermal and mechanical properties, was studied in the context of the thesis for its suitability for optical applications, especially as polymeric optical fibres.
However, processes known from the prior art for production of polyisocyanurate plastics from monomeric aliphatic diisocyanates have the fundamental disadvantage that a considerable shrinkage in volume occurs in the course of a trimerization reaction. Moreover, it is a common factor in the production processes for polyisocyanurate plastics which proceed from the monomeric diisocyanates and are known from the prior art that they are very time-consuming and take place in closed systems under complex temperature control.
WO 2015/166983 discloses the use of isocyanurate polymers for encapsulating LEDs. The use of carboxylates and alkoxides of alkali metals, alkaline earth metals or zirconium, in combination with complexing agents such as crown ethers or polyethylene glycols or polypropylene glycols, and organic tin compounds is not disclosed.
U.S. Pat. No. 6,133,397 only discloses coatings made by trimerizing oligomeric polyisocyanates. It does not disclose the production of solid bodies.
The problem addressed by the present invention was therefore that of providing a material which is transparent and has high thermal durability and thermal colour stability. In addition, the material should at the same time have low volume shrinkage and be producible by an efficient process.